One possibility is that you left a non-boot CD in the drive. That will cause it to cycle over and over as you mentioned.

My computer does the same thing, except that i can get to the start menu. After about 30 minutes or so, the computer restarts and whatever i didnt save, is lost.

It could possibly be a faulty power supply, or a memory problem. Or if it happens after the system has been on for a half hour or so, it could be the system fan.